Evolutionary anthropologist Robin Dunbar claims we can only cope with about 150 friends at a given time, 15 of whom could be considered intimate. And only five of those might be trustworthy.

We've embraced the idea of organizing our personal lives through social media in overwhelming numbers, while simultaneously developing the distinct impression it is bad for us.

Early this year I wrote an article defending Facebook, arguing that, rather than replace authentic connections with false ones, social media enables us to keep a lifetime of bonds alive, if we want it to. How can that be a bad thing?

It's a pleasing, sentimental logic I've clung to for years, until a recent New Yorker article by Maria Konnikova called "The Limits of Friendship" forced me to reassess, by highlighting work done over 20 years ago by an evolutionary anthropologist called Robin Dunbar.

By studying the social habits of primates in the late-'80s, Dunbar arrived at the theory that human beings have a limited capacity for intimacy that sets the number of friends we can have to an average of 150.

Beyond that number—which fluctuates between around 100 and 200, depending on how sociable the individual—we simply don't have the brainpower to maintain a meaningful connection without jettisoning one person for another in a sort mental "one in, one out" system.

The "Dunbar number," as it became known, then decreases by a rough rule of threes. So if there are 150 casual friends in your life you might invite to a large party, within that there are then 50 or so close friends you'd invite to a dinner party, 15 intimate friends you'd turn to for help or sympathy when you need it and five you'd trust with anything (usually close family and partners). At the other end of the scale you might have 500 acquaintances, or 1500 faces you could just about put a name to, but there will only ever be 150 people you can have any real emotional connection with at one time.

Dunbar soon found his theory was echoed in real life examples from around the world, as he explained when I spoke with him recently.

"A great case is the military," he says.

"All modern armies are based around companies with substructures that follows these numbers almost perfectly, a system they've arrived at through experience on the battlefield."

One hundred and fifty was also, Dunbar discovered, the estimated size of a Neolithic farming village, the splitting point of Hutterite settlements and the basic unit size of professional armies in Roman antiquity.

It is also, roughly, the average number of friends people have on Facebook.

"Facebook tries to force you to befriend people, because they're the friend of a friend of a friend, and people generously do up to a point, or at least until they get fed up," Dunbar says.

"But all they're doing is what we're doing in everyday life—they're tipping people over into a circle of acquaintances."

So the idea that social media allows us to maintain meaningful connections with people who have otherwise drifted out our lives is completely false?

"Yes and no," he says.

"It allows you to keep up with friends who you can't see so easily, and to some extent, that's preventing those friendships from decaying as they would naturally.

"But if you don't see those people, they will drop down through the circles and then off the end of your 150, and become an acquaintance—absolutely inevitably—because there's something quite special about face-to-face contact."

Few would argue, even in 2014. But why, exactly, is that the case?

Why isn't chatting online—where you can often be more focused, honest, and considered in what you say to each other than in a "real life" situation—not enough to keep you close to someone? Or why, for that matter, is enjoying a film together in the same room any more healthy than sharing and discussing it together online?

The interesting—if not entirely surprising—answer, according to Dunbar, is simple: touch.

We've all seen David Attenborough-narrated footage of primates picking fleas out of each other's necks to create social bonds. For all our greater sophistication of language and communication, we still overwhelmingly need that too.

"Just the way you casually touch someone on the arm or the shoulder when you're having a chat with them is so important," he says.

"It triggers the release of endorphins in the brain, and it's these that create this sense of warmth and calmness and contentedness with whoever you're doing it with."

Suddenly, it seems we're back in doomsday territory when it comes to most significant technological and societal change of the past decade.

With more and more of our interaction taking place online—and those of us who can remember when the world was any different slowly shuffling towards extinction—is social media not only failing to enhance our emotional bonds with each other, but threatening to leave our friendships permanently stunted and unfulfilled?

Despite this, it can't be entirely wrong to try and see some good in social media, if only because it isn't going anywhere fast.

Perhaps in our second decade on Facebook, we'll be less enthralled by it and more inclined to use it as a social tool, rather than a supplement, meaning a future where we're all quietly despairing cyborgs can still be avoided.

And anyway, as Dunbar himself insists, the technology itself still has a long way to go.

"The trouble is that it's unfair to bash social media completely, because for heaven's sakes, we've only just invented it," he points out at the end of our conversation.

"It's improving in quality and breadth almost by the day, so in five or 10 years time it may be a completely different kind of animal."

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